1. Field of the Invention
The present invention relates to an electrode arrangement for capacitive guard fences, and in particular to an electrode arrangement wherein the electrode has at least one electrical line wire stretched substantially parallel to the ground, and wherein a plurality of such electrodes are disposed in proximity to one another.
2. Description of the Prior Art
Guard fences are frequently employed for protecting an object, in addition to building protection, for preventing unauthorized penetration to the object, or at least to impede such unauthorized penetration. In order to perceive unauthorized penetration, capacitive guard fences are utilized which trigger an alarm when a trespasser approaches the fence or attempts to penetrate it, thus altering the quiescent capacitance. Installations having a need for security, such as, for example, nuclear power stations, military installations, and certain manufacturing establishments, require extremely reliable terrain protection systems.
Conventional capacitive guard fences generally exhibit a plurality of electrodes consisting of longitudinally stretched wires which are secured to generally vertical poles or supports with insulators at specific intervals. The capacitances between the individual electrodes or the capacitances to ground are measured and evaluated by means of processing circuitry, and an alarm or disturbance criterian is derived therefrom. Because such outdoor systems are particularly susceptible to disruption due to environmental influences, for example, swelling wires due to precipitation or a bird or other small animal perching on the electrode, than are interior protection systems disposed in closed buildings, various measures are undertaken in order to avoid false alarms, so-called illusory alarms. Such measures are generally realized in terms of circuit technology. The designer attempts to reduce illusory alarms to a minimum by means of complicated measurement and evaluation methods given a sufficiently high response sensitivity of the capacitive guard fence. These measures are, however, subject to limitation because the changes in capacitance which are caused, for example, by raindrops or by birds perching on the electrodes cannot be completely suppressed with mensuration and evaluation techniques.
A bird perching on an electrode wire, which usually exhibits a circular cross-section, effects a change in capacitance which decreases a percentage of the normal capacitance as the diameter of the wire increases. Precipitation such as rain, which collects on conventional electrode wires in the form of drops suspended next to one another, also effects a change in capacitance which decreases as a percentage of the normal capacitance as the wire diameter increases. Thus the effect of the above capacitance-changing causes can be minimized by simply increasing the wire diameters of the electrode wires. Such a possible solution, however, has the significant disadvantage of requiring rather heavy wires which present mechanical support problems. Moreover, even larger birds and animals may then perch on such wires.